Antennas are sensitive to the polarisation of electromagnetic wave and this is an important aspect of their operation.

Antenna polarisation is an important factor when designing and erecting antennas. Some antennas are vertically polarised, others vertical, and yet other antenna types have different forms of polarisation.

When designing an antenna, deciding on a particular form of antenna, or erecting an antenna, it is important to understand which way it needs to be polarised. Antennas with a particular polarisation will not be effective receiving electromagnetic wave signals with a different polarisation.

Antenna polarisation basics

For the electromagnetic wave the polarization is effectively the plane in which the electric wave vibrates. This is important when looking at antennas because they are sensitive to polarisation, and generally only receive or transmit a signal with a particular polarization.

For most antennas it is very easy to determine the polarization. It is simply in the same plane as the elements of the antenna. So a vertical antenna (i.e. one with vertical elements) will receive vertically polarised signals best and similarly a horizontal antenna will receive horizontally polarised signals.

It is important to match the polarization of the RF antenna to that of the incoming signal. In this way the maximum signal is obtained. If the RF antenna polarization does not match that of the signal there is a corresponding decrease in the level of the signal. It is reduced by a factor of cosine of the angle between the polarisation of the RF antenna and the signal.

Accordingly the polarisation of the antennas located in free space is very important, and obviously they should be in exactly the same plane to provide the optimum signal. If they were at right angles to one another (i.e. cross-polarised) then in theory no signal would be received.

For terrestrial radio communications applications it is found that once a signal has been transmitted then its polarisation will remain broadly the same. However reflections from objects in the path can change the polarisation. As the received signal is the sum of the direct signal plus a number of reflected signals the overall polarisation of the signal can change slightly although it remains broadly the same.

Polarisation categories

There are several categories of polarisation, and within each type there are several sub categories.

Linear polarisation: Linear polarisation is the most common form of antenna polarisation:

Horizontal polarisation: This form of antenna polarisation has horizontal elements. It picks up and radiates horizontally polarised signals, i.e. electromagnetic waves with the electric field in the horizontal plane.

Vertical polarisation: This form of antenna is typified by the vertical elements within the antenna. It could be a single vertical element. One of the reasons for using vertical polarisation is that antennas comprising of a single vertical element can radiate equally around it in the horizontal plane. Typically vertically polarised antennas have what is termed a low angle of radiation enabling a large proportion of their power to be radiated at an angle close to the earth’s surface. Vertically polarised antennas are also very convenient for use with automobiles.

Slant polarisation: This is a form of antenna polarisation that is at an angle to the horizontal or vertical planes. In this way both vertical and horizontally polarised antennas are able to receive the signal.

Circular polarisation: This has a number of benefits for areas such as satellite applications where it helps overcome the effects of propagation anomalies, ground reflections and the effects of the spin that occur on many satellites. Circular polarisation is a little more difficult to visualise than linear polarisation. However it can be imagined by visualising a signal propagating from an RF antenna that is rotating. The tip of the electric field vector will then be seen to trace out a helix or corkscrew as it travels away from the antenna.

Right hand circular polarisation: In this form of polarisation the vector rotates in a right handed fashion.

Left hand circular polarisation : In this form of polarisation the vector rotates in a left handed fashion, i.e. opposite to right handed.

Mixed polarisation: Another form of polarisation is known as elliptical polarisation. It occurs when there is a mix of linear and circular polarisation. This can be visualised as before by the tip of the electric field vector tracing out an elliptically shaped corkscrew.

It is possible for linearly polarised antennas to receive circularly polarised signals and vice versa. The strength will be equal whether the linearly polarised antenna is mounted vertically, horizontally or in any other plane but directed towards the arriving signal. There will be some degradation because the signal level will be 3 dB less than if a circularly polarised antenna of the same sense was used. The same situation exists when a circularly polarised antenna receives a linearly polarised signal.

Applications of antenna polarization

Different types of polarisation are used in different applications to enable their advantages to be used. Accordingly different forms of polarisation are used for different applications:

General radio communications: Linear polarization is by far the most widely used for most radio communications applications as the antennas are generally simpler and more straightforward.

Mobile communications: Vertical polarisation is often used for mobile radio communications. This is because many vertically polarized antenna designs have an omni-directional radiation pattern and it means that the antennas do not have to be re-orientated as positions as always happens for mobile radio communications as the vehicle moves.

Long distance HF ionospheric communications: Both vertical and horizontal polarisation are used:

Horizontal polarisation: Wire antennas are widely used for HF communications. These tend to be more easily erected using two poles leaving he wire antenna to be suspended between the two. In this way the antenna is horizontally polarised.

For large multi-element antenna arrays, mechanical constraints mean that they can be mounted in a horizontal plane more easily than in the vertical plane. This is because the RF antenna elements are at right angles to the vertical tower of pole on which they are mounted and therefore by using an antenna with horizontal elements there is less physical and electrical interference between the two.

Vertical polarisation: Antennas consisting of a single vertical element are widely used. The vertically polarised antenna provides a low angle of radiation which enables it to provide good long distance transmission and reception.

Satellite communications: Circular polarisation is sometimes used for satellite radio communications as there are some advantages in terms of propagation and in overcoming the fading caused if the satellite is changing its orientation.

As can be seen, each form of antenna polarisation has its own advantages which can be utilised to effect in particular instances.